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How do you Build a Magnet Detector

C

Cindy

Jan 1, 1970
0
I'm looking for a circuit that can detect a magnet passing through a
field about 1 foot away from the detector. The magnets are the small
powerful type as found in harddrives. We need to detect if a person
is carrying a magnet in his or her pocket or purse.

Is this possible?

Thanks,
Cindy
 
X

Xtrchessreal

Jan 1, 1970
0
I wish you luck on that.

FYI: You need to design around credit cards, discs, mp3 players, etc as
to not ruin a persons life.
 
R

Ron Hubbard

Jan 1, 1970
0
Try a battery connected to a resistor and a LED. Place a reed switch on
one of the supply lines. If a magnet comes by, the reed switch will
close and the LED will light. This is simple and effective if you can
get the circuit reasonably close to the magnets. If you build this,
make sure it's in a plastic case-- not metal.

Ron
 
J

Jordan

Jan 1, 1970
0
A magnetic compass might work?

Other extreme - a gauss meter.

Jordan
 
J

Jasen Betts

Jan 1, 1970
0
I'm looking for a circuit that can detect a magnet passing through a
field about 1 foot away from the detector. The magnets are the small
powerful type as found in harddrives. We need to detect if a person
is carrying a magnet in his or her pocket or purse.

Is this possible?

you could try a compass with wires either side of the needle, the magnet
deflects the needle and it contacts the wire... it worked for submerged
sea mines, it's not suitable for a handheld detector (and less than ideal
for portable) but may work for your aplication.

Bye.
Jasen
 
B

Bob Masta

Jan 1, 1970
0
I'm looking for a circuit that can detect a magnet passing through a
field about 1 foot away from the detector. The magnets are the small
powerful type as found in harddrives. We need to detect if a person
is carrying a magnet in his or her pocket or purse.

Is this possible?

Not sure what you mean by a magnet "passing through a field".
Do you already have some sort of magnetic field present?
To simply detect a moving magnet, you just need it to pass
by a coil of wire. The moving magnetic field of the magnet
will induce a curent in the coil, which you can detect. The
trick is making the setup sensitive enough. Having a soft
iron core in the coil will help. You may need to have an
unusual core shape, such that the face of the coil is
much taller than wide, in order to allow for people having
the magnet at different heights on their bodies. Or you
may want to use a column of some readily available
relay coils, appropriately wired in series.

Note that the magnet should pass perpendicular to
the axis of the coil.

Best regards,



Bob Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Home of DaqGen, the FREEWARE signal generator
 
S

spudnuty

Jan 1, 1970
0
Scientific American had an article in their Amateur Scientist some time
ago. Get it at the library or on their CD. The device detected very
minute variations in the earths magnetic field. As I remember it was
like an ultra-sensitive galvanometer. It used a tiny mirror on a
torsion thread in a magnetic field. The mirror had a laser pointer
bounced off it. I'm sure this thing could detect a magnet within 10
feet. A wire loop around a door frame and a sensitive enough detector
would also work.
This also gets into the area of violating someone's rights to privacy
and freedom from illegal search. Anyone scanned would need to be
warned.
Richard
 
C

Cindy

Jan 1, 1970
0
Thanks for all your replies. I guess I should have told you why... A
friend of mine works at a video store and they are losing a lot of
DVD's to someone who figured out the security lock device on the DVD
case. The security device that sets off the sensor at the door is
attatched to the DVD case, not the DVD. So, the thief unlocks the box
using 2 strong magnets, removes the DVD and makes a clean undetected
getaway. Knowing someone enters with a powerful magnet can give them
a heads up so they can keep an eye on the person.
 
R

Rich Webb

Jan 1, 1970
0
I'm looking for a circuit that can detect a magnet passing through a
field about 1 foot away from the detector. The magnets are the small
powerful type as found in harddrives. We need to detect if a person
is carrying a magnet in his or her pocket or purse.

Is this possible?

Very possible.

Look for info on "fluxgate magnetometers." There's a brief intro at
http://beale.best.vwh.net/measure/fluxgate/, lots of links at
http://www.sas.org/E-Bulletin/2004-04-02/wanderings/body.html, and some
relatively inexpensive (and easy to interface) gadgets at
http://fatquarterssoftware.bizland.com/store/page1.html. Also possible
to roll (wind) yer own.

It's probably even possible to mount the sensors in such a way as to
derive real-time X-Y positioning of the magnet.
 
R

Ron Hubbard

Jan 1, 1970
0
Add to the coil an op-amp with a gain of say ten, and it's output
connected to a transistor to act as a driver for a LED or a small
relay. While the magnet(s) moving through the coil will only produce a
short burst of current, the relay can be used to turn on a simple 555
oscillator to produce a tone of your choice.

Ron
 
J

Jasen Betts

Jan 1, 1970
0
Thanks for all your replies. I guess I should have told you why... A
friend of mine works at a video store and they are losing a lot of
DVD's to someone who figured out the security lock device on the DVD
case.

I got sold one with the device still attached, it took me a while to figure
out how to unlatch it. he hard drive magnet was at-hand fimly attached to
the refrigerator and did the trick easily, I was wondering if your application
was related.
The security device that sets off the sensor at the door is
attatched to the DVD case, not the DVD. So, the thief unlocks the box
using 2 strong magnets, removes the DVD and makes a clean undetected
getaway. Knowing someone enters with a powerful magnet can give them
a heads up so they can keep an eye on the person.

magnets are easy to shield from sensors.

Bye.
Jasen
 
R

Roger Dewhurst

Jan 1, 1970
0
Cindy said:
I'm looking for a circuit that can detect a magnet passing through a
field about 1 foot away from the detector. The magnets are the small
powerful type as found in harddrives. We need to detect if a person
is carrying a magnet in his or her pocket or purse.

Is this possible?

Very possible. Look at magnetometers. A fluxgate magnetometer would
certainly detect the field from such a magnet but you might be able to do it
much simpler technology. Look at metal detector circuits and other circuits
which detect currents induced in a loop or coil.

Someone might care to comment on the possibility of feeding the two ends of
a search coil into the + and- inputs of an op amp and detecting the output.

R
 
R

Roger Dewhurst

Jan 1, 1970
0
Cindy said:
Thanks for all your replies. I guess I should have told you why... A
friend of mine works at a video store and they are losing a lot of
DVD's to someone who figured out the security lock device on the DVD
case. The security device that sets off the sensor at the door is
attatched to the DVD case, not the DVD. So, the thief unlocks the box
using 2 strong magnets, removes the DVD and makes a clean undetected
getaway. Knowing someone enters with a powerful magnet can give them
a heads up so they can keep an eye on the person.

A coil constructed in or on the door frame will generate a current when a
magnet passes through it. Try an op-amp to detect the current in the coil.

R
 
C

cbm5

Jan 1, 1970
0
Cindy said:
Thanks for all your replies. I guess I should have told you why... A
friend of mine works at a video store and they are losing a lot of
DVD's to someone who figured out the security lock device on the DVD
case. The security device that sets off the sensor at the door is
attatched to the DVD case, not the DVD. So, the thief unlocks the box
using 2 strong magnets, removes the DVD and makes a clean undetected
getaway. Knowing someone enters with a powerful magnet can give them
a heads up so they can keep an eye on the person.


Have all entering customers roll in a pile of iron filings before they
are allowed to browse the selection.

Or, why not focus on the actual problem: one or more people who have
figured out your system, and are morally OK with stealing. It's probably
no more than a couple people. Buy or rent security cameras, taking note
of the criminal's video preferences. Once you have a tape of the
criminal in the act, alert the authorities to be on standby when you
spot that person again. Have them wait outside so the criminal can be
caught with the stolen items...problem is now solved.
 
Cindy said:
Knowing someone enters with a powerful magnet can give them
a heads up so they can keep an eye on the person.

Have you been building your own electronics stuff in the past? If not,

then this is probably too complicated for a total newbie. But a newbie
with lots of ambition might pull it off.

If I was doing this, I'd put a tiny chip of mirror on a compass, then
bounce the light from an LED off the mirror chip to a photodiode. Use
a
cheap lens to focus the LED to cast a small spot on the photocell, then

move things so the spot of light is only halfway on the photocell. If
the compass moves, the photocell output changes. Put it all in a
light-tight box. To make this more sensitive, just move the photocell
farther away from the compass so that a small compass deflection moves
the spot a larger amount (a larger box might be needed.)

For the mirror chip, I'd smash a cheap front-surface mirror bought from
a
surplus optics dealer. That, or try to put a silver or mercury plating
on a
microscope slide's cover-slip. Maybe put the whole compass in a cup
of kerosene or light mineral oil to keep it from wobbling from floor
vibrations.

Then rig up some kind of alarm circuit with a photodiode amplifier and
a "window comparator" that fires a piezo beeper whenever the light gets

brighter or dimmer.



A totally different method would be to amplify the output of a linear
Hall chip, then filter out the noise. Hall chips aren't too sensitive,

but they can just about detect a signal as strong as the Earth's field
(like a few Gauss.) This might detect a supermagnet from a couple
of feet away. Or it might not be sensitive enough. The main problem
is that the output voltage of a linear Hall chip will drift a bit, and
the
drift could mimic the magnet signal.

Linear Hall Chip $2.99
http://www.electronicsurplus.com/cc...ic-linear-hall-effect-sen-ugn3503u-135600.htm


Another method:
If you have a cheap oscilloscope, shut off the H deflection and turn
down the brightness. Use a tiny dot of opaque tape or foil to block
the little green glowing spot. Temporarily glue a photodiode or
photocell
over the location of the dot. Now if a magnetic field should cause
the electron beam to deflect, the green spot will come out from behind
the opaque dot and shine upon the photocell.

(((((((((((((((((( ( ( ( ( (O) ) ) ) ) )))))))))))))))))))
William J. Beaty SCIENCE HOBBYIST website
[email protected] http://amasci.com
EE/programmer/sci-exhibits amateur science, hobby projects, sci fair
Seattle, WA 206-789-0775 unusual phenomena, tesla coils, weird sci
 
J

Jasen Betts

Jan 1, 1970
0
For the mirror chip, I'd smash a cheap front-surface mirror bought from
a
surplus optics dealer. That, or try to put a silver or mercury plating
on a microscope slide's cover-slip.

or use a sequin, or a fragment of a blank part of a surplus cd.

Bye.
Jasen
 
R

Roger Dewhurst

Jan 1, 1970
0
Ron Hubbard said:
That's what I just said.

Ron

Do you suppose that a multi turn coil, about to metres in diameter,
shallowly buried under a driveway or path, if connected thus to an op-amp
would be sufficient to detect a firearm or crowbar carried over it? I
presume that such a coil would always detect a car?

R
 
H

Homer J Simpson

Jan 1, 1970
0
Thanks for all your replies. I guess I should have told you why... A
friend of mine works at a video store and they are losing a lot of
DVD's to someone who figured out the security lock device on the DVD
case.

As sold in the US most DVDs have a security device inside the retail case,
installed by the maker. Detector loops at the doors will alarm if the strip
is not cancelled by being magnetised.
 
R

Ron Hubbard

Jan 1, 1970
0
Yes, if you have enough gain. You can't get that kind of gain from a
single op-amp stage but two (or more) chained together will provide
sufficient gain without too much noise. You would also have to pick the
op-amps carefully to get good gain without excessive noise that could
drown out your incoming signal. Of course, you could probably do the
same thing with several transistor amplifier stages which needn't be
too complicated. If you look through some of the free circuitry
websites that have all kinds of circuits for alarms, games, etc., you
might find a circuit that you could easily adapt to your needs.

Ron
 
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